The present disclosure relates to a method for guiding a body that is tracking a target object, such that the tracking axis is prevented from pointing within a certain prohibited zone, while the duration of compromised tracking is minimized. In particular, the present disclosure relates to guiding a spacecraft.
It is often desirable for a spacecraft or some other body or vehicle in space, air, water or on land to track one or more objects, such as missiles, that are moving relative to the body, where tracking is defined as maintaining a body axis (such as the boresight of a sensing instrument) pointed at the object. In some of these cases, there may be additional constraints on the pointing of that axis, which supersede the goal of tracking. Such a case is the situation where it is desirable to avoid pointing a sensing instrument at a predetermined object or location. For example, a sensing instrument may be damaged if its boresight is pointed too close to the sun. In such a case, the boresight must be prevented from pointing within an exclusion zone, defined as the set of vectors that are less than a certain angle from the sun vector.
The target object may have a trajectory relative to the vehicle such that to track the target continuously, the boresight would have to pass through the exclusion zone. If the highest priority is to prevent the boresight from pointing into the exclusion zone, then the goal of tracking the target must be sacrificed temporarily in this situation. In robotic applications wherein the vehicle's guidance navigation and control (GNC) are handled autonomously without a human in the loop, there is desirably an automatic attitude (pointing direction, or angular orientation) guidance method to achieve the exclusion zone avoidance. This is not a trivial matter.
For example, a relatively simple method would be as follows: for each target-tracking attitude command that would point the boresight inside the exclusion zone, modify that command such that the boresight points along the vector which is as close as possible to the original vector while also being outside the exclusion zone. For a target trajectory requiring the boresight to pass through the exclusion zone, this method would result in a commanded attitude profile (a set of attitude commands over time) for which the boresight pointing approached the exclusion zone right up to the boundary, traced the boundary around to the point where the target-tracking pointing would exit the exclusion zone, and then resumed tracking. However, this method would not suffice. The actual vehicle has dynamic limits, that is, limits on its capabilities for angular rates and angular accelerations. The method described above could cause high commanded rates, and would certainly cause high commanded accelerations at the first and last intersections with the exclusion zone boundary, exceeding the vehicle's dynamic limits. Then, while the vehicle's feedback control system would attempt to make the vehicle follow the commands, it would not be able to, and the boresight would enter the exclusion zone.
To successfully avoid the exclusion zone, the avoidance maneuver must be initiated before the point where the tracking pointing would enter the exclusion zone. Also, the faster the tracking pointing is approaching the exclusion zone, the further in advance the maneuver must be initiated in order to generate a pointing profile that avoids the exclusion zone, but remains within the vehicle's dynamic limits. So then, another method could be designed with a conservative strategy such that the boresight avoided the exclusion zone always using a pointing path that would remain within dynamic limits for the maximum possible approach rate (the maximum possible angular rate at which the tracking pointing could approach the exclusion zone), and thus for all possible approach rates. However, this is an unfavorable strategy because although the highest priority is to keep the boresight out of the exclusion zone, it is also a high priority to minimize the duration of compromised tracking. The conservative method described above would cause unnecessarily long durations of compromised tracking in situations where the tracking pointing crossed the exclusion zone with approach rates lower than the maximum possible.
Also, a predictive method could be designed which takes the current situation and extrapolates it into the future, propagating forward the vehicle's and target's relative positions. Then, with the predicted tracking pointing profile, the method would design an optimal avoidance profile. This is unfavorable due to its high complexity and high computational requirements.